Currently, many mobile telephones use one or more of the following three frequency bands: the GSM band centred on the frequency 900 MHz, the DSC band centred on 1800 MHz, and the PCS band centred on 1900 MHz. The 900 MHz and 1800 MHz frequency bands are separated by one octave, whereas the 1800 MHz and 1900 MHz frequency bands are separated by only a fraction of one octave. In many mobile telephones using the 900 MHz and 1800 MHz frequency bands, the antenna has separate portions tuned to respective ones of the two frequency bands, since it is not considered feasible to have one and the same portion of the antenna tuned to a frequency band of more than one octave, with a relatively large unused frequency band between the useful frequency bands.
U.S. Pat. No. 5,512,910 describes a microstrip antenna device having three resonance frequencies. However, an antenna of this type is too large to be used conveniently in a small mobile phone.
A known dual band antenna, as shown in U.S. Pat. No. 6,166,694, has a conductor portion, from which two spirals branch off. The two spirals are tuned to form a high band portion and a low band portion.
European Patent Application No. 00610112.5 (not published, and not forming part of the state of the art) describes an antenna of this type, housing a second conductor, which is capacitively coupled to the first conductor, and tuned to a second high frequency band.
It is the object of the invention to provide an antenna, which is usable in at least three frequency bands and which has the smallest possible loss, that is the maximum possible gain, in all frequency bands.